In a typical optical spectrometer, light is taken in and focused onto an entrance aperture, typically a slit, and a portion of the light passes through this aperture. The light beam passing through the entrance aperture then encounters a collimating element to produce a collimated beam of light rays that are parallel to each other. The collimated light beam is then projected onto a dispersive element such as a diffraction grating or a prism which split the input light beam into its spectral components in the form of multiple collimated, parallel, light beams traveling at different angles depending on the wavelength of light prescribed by the grating equation. The multiple output light beams from the dispersive element encounter a focusing element and are then focused onto either a linear or an array detector, such as a charged-coupled device (CCD) detector or a complementary metal oxide semiconductor (CMOS) detector or Indium Gallium Arsenide (InGaAs), where the light beams are recorded as a spectrum.
The spectral resolution and signal-to-noise ratio (SNR) of a typical optical spectrometer, which affects the ability to distinguish closely spaced spectral features such as transmission lines, absorption lines, and emission lines, is controlled by a number of different factors such as the size and shape of the entrance aperture, the optical characteristics of the dispersive element, the optical characteristics of the collimating optics and the focusing optics, and the size and shape of the detector's pixels. In particular, the size of the entrance aperture (e.g., width of slit used as entrance aperture) and the optical power of the focusing element are the primary factor affecting the tradeoff between spectral resolution and SNR. A focusing element that can create a very sharp focused spot can be useful in increasing the spectral resolution of a spectrograph but, there exists fundamental limitations in manufacturing and designing of focusing elements with high optical powers (small focal ratio f/# or large numerical aperture) therefore more efforts have been put forth in decreasing the size of the entrance aperture in order to increase the spectral resolution. The spectral resolution of the spectrometer can be increased by decreasing the size, specifically its width along the spectral axis, of the entrance aperture, as a smaller portion of the light taken in by the spectrometer passes through the entrance aperture and therefore subtends a smaller portion of the detector array to allow closely spaced spectral features to be distinguished. However, this decrease in size of the entrance aperture also decreases the light throughput of the spectrometer as less light is allowed to enter the spectrometer, and therefore leads to a decrease in SNR. In applications such as Raman spectroscopy, biomedical spectroscopy, and astronomy among many other applications, where the amount of light available to the spectrometer is low and signal quality is paramount, a system, method, and/or apparatus that allows for an increase in both spectral resolution and SNR is highly desired.
Previous methods of improving both the spectral resolution and SNR has focused primarily on the design of analog optical slicers situated before the dispersive element of the spectrometer. Some optical slicers use specialized prisms to slice a light beam [1], where the performance depends on the optical properties of the prism which is wavelength dependent and can limit its use in broadband light conditions. Some optical slicers -often referred to as integral field slicers- make use of slicer mirror arrays, lenslet arrays, or fiber optical bundles, in the image space, to redirect portions of the light to their respective spectrometer, thus slicing the light beam into portions with at least one of the spatial dimensions smaller than the received light beam. However, such optical slicers can be large in size and limited in getting high spectral information from all the different beam portions [2, 3, and 4]. In [5], a pupil-based optical slicer is introduced, comprising of a beam reformatter and at least one of a beam compressor and a beam expander, to improve spectral resolution while allowing for high throughput via a large entrance aperture by negating the use of a slit. The beam reformatter receives a full aperture of collimated light beam and splits it into two or more beam portions where at least one of the spatial dimensions is smaller than the received original light beam, and propagates the beam portions in the same direction to form a reformed composite beam containing the same spectral information as the received beam but with one of the spatial dimensions smaller than the received light beam. A beam expander, if used, receives the reformed beam from the beam reformed and produces an expanded beam. Abeam compressor, if used, receives the light beam first and produces a compressed light beam that passed into the beam reformed. While such optical slicing methods do improve both spectral resolution and SNR, they come at the expense of the introduction of additional analog optical elements that increase not only the complexity of the spectrometer, but also increases the cost of the spectrometer as well. These optical components (e.g., lenses, reflective surfaces, etc.) along with the associated mounting apparatus will introduce aberrations and other performance issues (i.e. alignment) to the device. Therefore, a system, method, and/or apparatus that allows for an increase in both spectral resolution and SNR without the introduction of additional analog optical components and associated mounting apparatus, such as that in an optical slicer, to the spectrometer is highly desired. Furthermore, a system, method, and/or apparatus that can further increase the spectral resolution and SNR of the spectrometer in conjunction with an optical slicer is also highly desired.